Okay, this should be easy, as I know I must be overlooking something simple.

I upgraded my site to DNN 4.8 and then installed .Net Framework 3.5 on my server.

To test out the 3.5 Framework, I added a new asp:ListView Control to a module, which of course added the following declaration at the top of ascx control:
@ Register Assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" Namespace="System.Web.UI.WebControls" TagPrefix="asp" 

After navigating to a page this module is on, my web.config was automatically modified and my site crash with the following error:
'ScriptServiceAttribute' is ambiguous in the namespace 'System.Web.Script.Services' (which was referenced from a webservice I have in my App_Code directory.)

I understand the error is a conflict of namespace between System.Web.Extensions, Version=3.5x and System.Web.Extensions, Version=3.1x but how do I correct the errors?

Do I just remove one of the following lines from the web.config?
add assembly ="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/ 
add assembly ="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/

Or is there documentation on how to upgrade your Dotnetnuke modules code to the 3.5 framework?

Thanks for any help anyone can give, as I couldn't find any documentation elsewere...